Particle Physics Masterclass 2014


Once again Rooks Heath A level students were lucky to take part in this study day. Unfortunately we weren’t able to complete all the activities but we had a brilliant time.

Our course took part at the Rutherford Appleton laboratory based in Harwell, Oxfordshire.



Aerial View of the Harwell Science Innovation Campus, including the Rutherford-Appleton Laboratory, The Diamond light source is in the background.

We welcomed to the course by Jo Lewis and Sophy Palmer, who outlined what, was going to happen during the day.

A very brief guide to Accelerators

Dr. Emmanuel Olaiya


The lecture began by looking at the history of probing matter. How four hundred years ago the only apparatus available were light microscopes and telescopes. Light microscopes looked at the very small and light telescopes looked at the very large, but from a great distance.

Robert Hooke FRS (July 1635 – 3 March 1703) was an English natural philosopher, architect and polymath (an all-round clever clogs).


He was one of the first people to take microscopy seriously. In 1665 he published Micrographia, a book describing observations made with microscopes and telescopes, as well as some original work in biology. He coined the term cell for describing biological organisms, the term being suggested by the resemblance of plant cells to monks’ cells. The hand-crafted, leather and gold-tooled microscope he used to make the observations for Micrographia (see the pictures above), was originally constructed by Christopher White in London. It is now on display at the National Museum of Health and Medicine in Washington, DC.

Cell structure of cork drawn by Hooke can be seen below left.


Above right is Hooke’s drawing of a flea

At the other end of the scale from the microscope is the telescope.

Galileo Galilei (5 February 1564 – 8 January 1642) was an Italian physicist, mathematician, engineer, astronomer, and philosopher who played a major role in the scientific revolution.

A replica of Galileo’s telescope


Galileo didn’t invent the telescope but he did improve the design and was able to use it to discover Jupiter.

These pieces of equipment were brilliant in their day but we need far more sensitive equipment now.

For the large scale of space we have the Hubble telescope.

The Hubble Space Telescope (HST) is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation.

The Hubble Deep Field (HDF) is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope.


In 2018 the Hubble telescope will be succeeded by the James Webb telescope.

For the small scale we have the Large Hadron Collider

The Large Hadron Collider (LHC) is the world’s largest and most powerful particle accelerator. It first started up on 10 September 2008, and remains the latest addition to CERN’s accelerator complex. The LHC consists of a 27-kilometre ring of superconducting magnets with a number of accelerating structures to boost the energy of the particles along the way.


Electromagnetic waves can be used to do the probing and the shorter wavelengths allow us to look at smaller objects.


The higher the energy of the electromagnetic wave the shorter its wavelength and the smaller the object it can probe.


Louis-Victor-Pierre-Raymond, 7th duc de Broglie, (15 August 1892 – 19 March 1987) was a French physicist who made ground-breaking contributions to quantum theory.

He suggested there was a connection between wave and particle nature. A moving particle always has wave associated with it and the particle is controlled by the wave. He suggested that light has dual nature.

l = h/p

There are three symbols in this equation:

a) l stands for the wavelength of the particle

b) h stands for Planck’s Constant

c) p stands for the momentum of the particle.

So particles can also do the probing. An example of this is electron diffraction.


Typical electron diffraction pattern obtained in a TEM (Transmission electron microscope) with a parallel electron beam

De Broglie’s equation can be arranged as


Where m is the mass of the particle (e.g. an electron), e is the charge and V is the accelerating potential difference. Increasing the potential difference increases the energy of the particle. Therefore increasing the energy gives a particle with a shorter wavelength, which can be used to probe very small things.

Very high energy particles can be found in cosmic waves, which mainly originate outside the solar system. They are composed primarily of high-energy protons and atomic nuclei.


Compilation of measurements of the energy spectrum of charged cosmic rays. The observations can be described by a power-law with spectral breaks at 4 PeV, referred to as the knee, a second knee at 400 PeV and the ankle at 1 EeV.

The energy spectrum of cosmic rays has been measured up to 10^21 eV (electron-volts) and is shown below. At the highest energies, cosmic rays have roughly the same energy as a well-struck tennis ball, but packed into a single atomic nucleus.

The energy spectrum of cosmic rays has been relatively well-studied up to 10^18 eV. The change in the spectral index is called the “knee.”

Breaks in the cosmic ray spectrum are thought to be correlated with changes in the composition and sources of the particles. It is believed that below the “knee” most cosmic rays are protons accelerated in supernova remnants inside our Galaxy. In this picture, the decrease in flux at the “knee” can be interpreted as a change in the magnetic confinement of the cosmic rays. As their energies increase, the cosmic rays above a critical energy can escape the magnetic fields in the Galaxy.

Our current picture of cosmic rays below the “knee” — acceleration of particles in supernova remnants — can explain the power law spectrum observed in the cosmic ray flux.


The Earth and its atmosphere are hit by elementary particles and atomic nuclei of very large energies. Most of them are protons (hydrogen nuclei) and all sorts of nuclei up to uranium (although anything heavier than nickel is very, very rare). Those are usually meant when talking about cosmic rays. Other energetic particles in the cosmos are mainly electrons and positrons, as well as gamma-rays and neutrinos.

The cosmic rays will hardly ever hit the ground but will collide (interact) with a nucleus of the air, usually several ten kilometres high. In such collisions, many new particles are usually created and the colliding nuclei evaporate to a large extent.

Most of the new particles are pi-mesons (pions). Neutral pions very quickly decay, usually into two gamma-rays. Charged pions also decay but after a longer time. Therefore, some of the pions may collide with yet another nucleus of the air before decaying, which would be into a muon and a neutrino. The fragments of the incoming nucleus also interact again, also producing new particles.

The gamma-rays from the neutral pions may also create new particles, an electron and a positron, by the pair-creation process. Electrons and positrons in turn may produce more gamma-rays by the bremsstrahlung mechanism.





The interactions in the atmosphere are difficult to identify and difficult to control. That is why we need accelerators if we want the high energy particles.

In accelerators magnetic fields are used to contain and control the movement of charged particles.

Forces on charged particles is given by the Lorentz force

The Lorentz force is the combination of electric and magnetic force on a point charge due to electromagnetic fields. If a particle of charge q moves with velocity v in the presence of an electric field E and a magnetic field B, then it will experience a force. For any produced force there will be an opposite reactive force. In the case of the magnetic field, the reactive force may be obscure, but it must be accounted for.

F = qE + qv x B

qE provides electric field acceleration (below left) and B provides magnetic field bending (below right)


The direction of the magnetic part of the force is given by the right hand rule. Fleming’s left hand rule is the version of it that applies to charged particles in conductors.


For a free charged particle (q) F = qvBsinq and F is perpendicular to the plane of v (velocity) and B (magnetic field)


To determine the direction of the magnetic force on a positive moving charge, you point the thumb of the right hand in the direction of v, the fingers in the direction of B, and a perpendicular to the palm points in the direction of F. One way to remember this is that there is one velocity, and so the thumb represents it. There are many field lines, and so the fingers represent them. The force is in the direction you would push with your palm. The force on a negative charge is in exactly the opposite direction to that on a positive charge.

So what is the link between accelerators and the structure of the atom?

Common to all accelerators is the use of electric fields for the acceleration of charged particles; however, the manner in which the fields are applied varies widely. The most straightforward type of accelerator results from the application of a potential difference between two terminals. To obtain more than about 200 kV of accelerating voltage, it is necessary to use one or more stages of voltage-doubling circuits. The first such device was built by J. D. Cockcroft and E. T. S. Walton in 1932 and was used for the first transmutation experiments with artificially accelerated particles (protons). Cockcroft-Walton accelerators are still widely used today, sometimes as injectors to much larger accelerators.

For their work, Cockcroft and Walton won the Nobel Prize in Physics for splitting the atomic nucleus and they were instrumental in the development of nuclear power.

1932 saw the announcement of the first apparatus for artificially accelerating atomic particles to high energies: the Cockcroft-Walton accelerator. And, barely a month later, beams of high-energy protons produced by this machine were used to initiate the disintegration of lithium nuclei, and thereby confirm the equivalence of mass and energy.

Nature 129, 242 & 649 (1943)



The first apparatus to produce an artificial nuclear disintegration by bombarding lithium with accelerated protons (published 1932).

Even though the first linear particle accelerator was patented in 1928 by Rolf Widerøe, who also built the first operational device the linac is considered to be a later, great improvement on Cockcroft and Walton accelerator.

A linear particle accelerator (often shortened to linac) is a type of particle accelerator that greatly increases the velocity of charged subatomic particles or ions by subjecting the charged particles to a series of oscillating electric potentials along a linear beamline; this method of particle acceleration was invented by Leó Szilárd.

In a linear accelerator (LINAC) the charged particles are accelerated in a straight line. The diagram below shows the principle of operation of a LINAC.


An alternating p.d. is connected across adjacent cylindrical electrode tubes. Charged particles are accelerated across the gaps between electrodes. By the time the particle reaches the next gap the polarity of electric field has reversed and so the particle is accelerated once more. Each of these little steps increases the energy of the particles. Because the charged particles need to spend the same amount of time in each tube, the tubes become increasingly longer because the speed of the particles has increased.

LINACs have an important advantage over circular accelerators. When a charged particle is moved in a circular path it radiates energy (synchrotron radiation) so that a lot of the input energy is wasted. There is no such waste in a linear accelerator. However, the length of a LINAC limits the energy achieved.

Founded in 1962 as the Stanford Linear Accelerator Centre, the facility is located on 1.72 square kilometres of Stanford University-owned land on Sand Hill Road in Menlo Park, California—just west of the University’s main campus. The main accelerator is 3km miles long—the longest linear accelerator in the world—and has been operational since 1966. It is also one of the most powerful linear accelerators and can accelerate electrons up to an energy of 50 GeV.

As well as linear accelerators there are also circular accelerators. The first was a cyclotron. A cyclotron is a type of particle accelerator in which charged particles accelerate outwards from the centre along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying (radio frequency) electric field. It was invented and patented by Ernest Lawrence of the University of California, Berkeley, where it was first operated in 1932. It could produce energies of up to 80keV.


The diagram above is of the cyclotron operation from Lawrence’s 1934 patent. The “D” shaped electrodes are enclosed in a flat vacuum chamber, which is installed in a narrow gap between the two poles of a large magnet.

Cyclotrons accelerate charged particle beams using a high frequency alternating voltage which is applied between two “D”-shaped electrodes (also called “dees”). An additional static magnetic field B is applied in perpendicular direction to the electrode plane, enabling particles to re-encounter the accelerating voltage many times at the same phase. To achieve this, the voltage frequency must match the particle’s cyclotron resonance frequency


with the relativistic mass m and its charge q. This frequency is given by equality of centripetal force and magnetic Lorentz force. The particles, injected near the centre of the magnetic field, increase their kinetic energy only when recirculating through the gap between the electrodes; thus they travel outwards along a spiral path. Their radius will increase until the particles hit a target at the perimeter of the vacuum chamber, or leave the cyclotron using a beam tube.

TRIUMF, Canada’s national laboratory for nuclear and particle physics, houses one of the world’s largest cyclotrons. The 18 m diameter, 4,000 tonne main magnet produces a field of 0.46 T while a 23 MHz 94 kV electric field is used to accelerate the 300 μA beam. The TRIUMF field goes from 0 to about 8.13m radius with the maximum beam radius of 8.13m. This is because it requires a lower magnetic field to reduce EM stripping of the loosely bound electrons. Its large size is partly a result of using negative hydrogen ions rather than protons. The advantage is that extraction is simpler; multi-energy, multi-beams can be extracted by inserting thin carbon stripping foils at appropriate radii. TRIUMF is run by a consortium of eighteen Canadian universities and is located at the University of British Columbia, Vancouver, Canada.


A synchrotron is a particular type of cyclic particle accelerator originating from the cyclotron in which the guiding magnetic field (bending the particles into a closed path) is time-dependent, being synchronized to a particle beam of increasing kinetic energy. The synchrotron is one of the first accelerator concepts to enable the construction of large-scale facilities, since bending, beam focusing and acceleration can be separated into different components. A fixed magnetic field keeps particles moving in a circle. The size of the magnet is the limiting factor in a cyclotron.

Edwin McMillan constructed the first electron synchrotron in 1945, although Vladimir Veksler had already (unknown to McMillan) published the principle in a Soviet journal in 1944. The first proton synchrotron was designed by Sir Marcus Oliphant and built in 1952

It is the most recent and most powerful member of the accelerator family. A synchrotron consists of a tube in the shape of a large ring through which the particles travel; the tube is surrounded by magnets (quadropoles) that keep the particles moving through the centre of the tube (focus the beam). The particles enter the tube after already having been accelerated to several million electron volts. Particles are accelerated at one or more points on the ring each time the particles make a complete circle around the accelerator. To keep the particles in a rigid orbit, the strengths of the magnets in the ring are increased as the particles gain energy. In a few seconds, the particles reach energies greater than 1 GeV and are ejected, either directly into experiments or toward targets that produce a variety of elementary particles when struck by the accelerated particles. The synchrotron principle can be applied to either protons or electrons, although most of the large machines are proton-synchrotrons.


Examples of synchrotrons:

Super Proton (SPS), high vacuum and a 6km circumference

Diamond light source

The Large Electron–Positron Collider (LEP) was one of the largest particle accelerators ever constructed.

It was built at CERN, a multi-national centre for research in nuclear and particle physics near Geneva, Switzerland. LEP was a circular collider with a circumference of 27 kilometres built in a tunnel roughly 100 m underground and passing through Switzerland and France. It was used from 1989 until 2000. Around 2001 it was dismantled to make way for the LHC, which re-used the LEP tunnel. To date, LEP is the most powerful accelerator of leptons ever built.

As in all ring colliders, the LEP’s ring consisted of many magnets which forced the charged particles into a circular path (so that they stayed inside the ring), RF accelerators which accelerated the particles with radio frequency (RF) waves, and quadrupoles that focused the particle beam (i.e. keep the particles together). The function of the accelerators was to increase the particles’ energies so that heavy particles could be created when the particles collided. When the particles were accelerated to maximum energy (and focused to so-called bunches), an electron and a positron bunch was made to collide with each other at one of the collision points of the detector. When an electron and a positron collided, they annihilated to a virtual particle, either a photon or a Z boson. The virtual particle almost immediately decayed into other elementary particles, which were then detected by huge particle detectors.

Electromagnetic waves can accelerate particles in the same way that water waves push surfers. The timing is vital.

The Tevatron was a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory (also known as Fermilab), just east of Batavia, Illinois, and holds the title of the second highest energy particle collider in the world after the Large Hadron Collider (LHC) near Geneva, Switzerland. The Tevatron was a synchrotron that accelerated protons and antiprotons in a 6.86 km ring to energies of up to 1 TeV, hence its name.

A synchrotron has a varying magnetic field and provides kicks of energy. Blue magnets control the focussing (quadrapoles) and the red magnets control the trajectories (Dipole). There are also corrector magnets.

Normally synchrotron radiation is considered a nuisance but Diamond uses it.

The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets.. It is similar to cyclotron radiation except that synchrotron radiation is generated by the acceleration of ultrarelativistic charged particles through magnetic fields. Synchrotron radiation may be achieved artificially in synchrotrons or storage rings, or naturally by fast electrons moving through magnetic fields. The radiation produced in this way has a characteristic polarization and the frequencies generated can range over the entire electromagnetic spectrum.


Synchrotron radiation from a bending magnet

A synchrotron light source is a source of electromagnetic radiation (EM) usually produced by a storage ring, for scientific and technical purposes. First observed in synchrotrons, synchrotron light is now produced by storage rings and other specialized particle accelerators, typically accelerating electrons. Once the high-energy electron beam has been generated, it is directed into auxiliary components such as bending magnets.

The major applications of synchrotron light are in condensed matter physics, materials science, biology and medicine. A large fraction of experiments using synchrotron light involve probing the structure of matter from the sub-nanometre level of electronic structure to the micrometre and millimetre level important in medical imaging. An example of a practical industrial application is the manufacturing of microstructures by the LIGA process.

Diamond Light Source is the UK’s national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.

The facility is used by over 3,000 academic and industrial researchers across a wide range of disciplines including structural biology, energy, engineering, nanoscience and environmental sciences.

The loss of energy per orbit can be calculated from


This energy is in the form of gamma radiation which is sued for studies at the atomic/molecular level,

The future of accelerators is the international linear collider.

The International Linear Collider (ILC) is a proposed linear particle accelerator. It is planned to have a collision energy of 500 GeV initially, with the possibility for a later upgrade to 1000 GeV (1 TeV). The host country for the accelerator has not yet been chosen and proposed locations are Japan, Europe (CERN) and the USA (Fermilab). Japan is considered the most likely candidate, as the Japanese government is willing to contribute half of the costs, according to the coordinator of study for detectors at the ILC . Construction could begin in 2015 or 2016 and will not be completed before 2026.

Studies for an alternative project called CLIC the Compact Linear Collider are also underway, which would operate at higher energies (up to 3 TeV) in a machine with comparable length as the ILC.

The ILC would collide electrons with positrons. It will be between 30 km and 50 km long, more than 10 times as long as the 50 GeV Stanford Linear Accelerator, the longest existing linear particle accelerator. The proposal is based on previous similar proposals from Europe, the U.S., and Japan.

Fundamentals of Particle Physics – Particles and forces


Professor Bill Scott

There are 12 fundamental matter particles (24 matter particles if you count quark colours, 3 forces (if you ignore gravity because it is so weak) and 12 fundamental force particles (if you ignore the graviton). The Higgs has now been accepted as existing.

The 12 fundamental particles are made up of 6 quarks (3 up and 3 down), 3 charged leptons and 3 neutrinos.

The 3 forces are the strong, weak and electromagnetic forces.

The 12 force fundamental particles are 8 gluons, 3 weakons and 1 photon.

The smallest particle we are all familiar with is the atom.


The mass equivalent of the fundamental negatively charged electron is 0.511MeV.

The mass equivalent of the positive proton is 938.3 MeV and the neutral neutron is 939.6 MeV.


1eV = 1.6 x 10^-19J

Quantum mechanics relies on the fact that particles can have wave like behaviour and that waves can have particle like behaviour. Einstein said electromagnetic waves can actually be considered as packets of energy called photons. The energy of a photon E = hf where h is Planck’s constant and f is the frequency in time.

De Broglie produced a formula that linked a particle’s momentum to its De Broglie wavelength and frequency, P = hk where P is the momentum, h is Planck’s constant and k is the frequency in space. K = 1/l


The top wave has “high” momentum and the bottom wave has ”low” momentum

Heisenberg’s uncertainty principle says that the probability of finding a particle is the “wave”. You either know the position of the particle or know its momentum (and vice versa. You can’t know both. This is the uncertainty.

In 1927, Werner Heisenberg stated that the more precisely the position of some particle is determined, the less precisely its momentum can be known, and vice versa.


The nucleus of the atom is made up of protons and neutrons, together classed as nucleons. Nucleons are composite because they are made up of three quarks.

A proton consists of two up quarks, one down quark and a gluon (these particles are fundamental). Similarly a neutron consists of two down quarks, one up quark and a gluon.

The uncertainty principle says that the quarks must be bouncing about. They move very fast where the wave of the particle comes from kinetic energy. The kinetic energy gives the quarks mass.

How do we know that there are quarks inside the nucleon? We can do electron-quark “scattering” and see. (e.g. at the HERA electron-proton collider)


Photons (particles of em radiation with 0 mass) are responsible for transferring electrical and magnetic forces. Explained by photon exchange and Feynman diagrams are used to illustrate the processes. Below left is the Feynman diagram for the electron-quark scattering. The g is a virtual photon. Feynman diagram for beta decay is below right. In beta decay a negatively charged down quark is converted into a positively charged up quark by emission of a W− boson; the W− boson subsequently decays into an electron and an electron antineutrino. The “weak” force is involved in beta decay.



In theoretical physics, Feynman diagrams are pictorial representations of the mathematical expressions governing the behaviour of subatomic particles. The scheme is named for its inventor, Nobel Prize-winning American physicist Richard Feynman, and was first introduced in 1948. The interaction of sub-atomic particles can be complex and difficult to understand intuitively, and the Feynman diagrams allow for a simple visualization of what would otherwise be a rather arcane and abstract formula.

Quantum electrodynamics, commonly referred to as QED, is a quantum field theory of the electromagnetic force. Taking the example of the force between two electrons, the classical theory of electromagnetism would describe it as arising from the electric field produced by each electron at the position of the other. The force can be calculated from Coulomb’s law. The quantum field theory approach visualizes the force between the electrons as an exchange force arising from the exchange of virtual photons. It is represented by a series of Feynman diagrams, the most basic of which is


With time proceeding upward in the diagram, this diagram describes the electron interaction in which two electrons enter, exchange a photon, and then emerge. Using a mathematical approach known as the Feynman calculus, the strength of the force can be calculated in a series of steps which assign contributions to each of the types of Feynman diagrams associated with the force.

QED applies to all electromagnetic phenomena associated with charged fundamental particles such as electrons and positrons, and the associated phenomena such as pair production, electron-positron annihilation, Compton scattering, etc.

H1 is a particle detector in operation at HERA (Hadron Elektron Ring Anlage) in DESY, Hamburg. It began operating together with HERA in 1992. Leptons (electrons or positrons) are collided with protons by HERA in the interaction point of H1. H1 is operated by an international collaboration of about 400 physicists from 42 institutes in 15 countries. The detector is about 12 x 15 x 10 meters big and weighs 2800 tons. It is accompanied by an electronics trailer three stories high. It took data until the shutdown of HERA in June 2007.

It was designed for the decryption of the inner structure of the proton, the exploration of the strong interaction as well as the search for new kinds of matter and unexpected phenomena in particle physics.

DESY lab in Hamburg




30GeV of electrons collide with 900 GeV. A struck quark within the proton forms a “jet” of mesons and electrons are scattered.

Quarks and gluons are said to be “confined” in hadrons. A meson is a composite particle made up of a quark and antiquark.

Confinement is a property of the strong force. The strong force works by gluon exchange but at “large” distance the self-interaction of the gluons breaks the inverse-square law forming “flux – tubes”. This means pulling quarks apart far enough breaks the inverse square law.


Since the strong force increases as quarks move apart, they can only get so far…
The quarks are confined together inside hadrons.
Hadron jail!


Think of the two charm quarks as being connected by a tight spring.
As the spring stretches, the energy stored in it increases. If you keep on pulling, eventually, the spring “snaps”.
In the case of quarks, as they separate, the energy stored in the “gluon spring”
increases until it eventually “snaps”. When it does, the energy stored in this
spring is converted into mass in the form of a quark-antiquark (down-antidown quarks) pair.
So, the initial charm-anticharm state has now become two particles, a charm-antidown + a down-anticharm.

An example of conversion of energy into mass!

Also note that the strong interaction can only produce quark-antiquark pairs of
the same type. That is (down-antidown, up-antiup, strange-antistrange, etc)

Quarks and gluons carry “colour” – quantum numbers analogous to electric charge, but only “colourless” objects like baryons (3-quark states such as protons) and mesons (quark-antiquark states) escape confinement.

Quantum “chromo” Dynamics (QCD)

In theoretical physics, quantum chromodynamics (QCD) is a theory of strong interactions, a fundamental force describing the interactions between quarks and gluons which make up hadrons such as the proton, neutron and pion. QCD is a type of quantum field theory called a non-abelian gauge theory with symmetry group SU(3). The QCD analogue of electric charge is a property called ‘colour’. Gluons are the force carrier of the theory, like photons are for the electromagnetic force quantum electrodynamics. The theory is an important part of the Standard Model of particle physics. A huge body of experimental evidence for QCD has been gathered over the years.

Quarks come in three “colours” – red, blue and green.

Quarks interact with each other by the exchange of gluons; a primitive vertex in the Feynman diagram involves a change in “colour”.

The gluons carry the “colour charge” and therefore the emergent quark will not have the same colour as the entering quark. This process is very different from the electromagnetic force since the photon as the exchange particle for the force between charges does not itself carry charge.

The interaction depicted below is responsible for binding quarks together into mesons and baryons, and responsible for holding protons and neutrons together to form nuclei. The gluon mediates the interaction between two quarks.



Gluons come in eight “colours”. The force particles can scatter each other Gluons can bounce off other gluons. Gluons are not really coloured. The colour names are just used to differentiate them.

Gluons are the exchange particles for the colour force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles. The gluon can be considered to be the fundamental exchange particle underlying the strong interaction between protons and neutrons in a nucleus. That short-range nucleon-nucleon interaction can be considered to be a residual colour force extending outside the boundary of the proton or neutron. That strong interaction was modelled by Yukawa as involving an exchange of pions, and indeed the pion range calculation was helpful in developing our understanding of the strong force.

The Quark/gluon in UA1 P P experiment (CERN 1981 – 1987) is famous for the discovery of the W and Z boson – the carriers of the “weak” force



The free neutron is an unstable particle and lasts only about 15 minutes. It beta decays to a proton with the emission of an electron (e-) and an (anti-) neutrino. At the level of the quarks, a d-quark in the neutron is changing into an u-quark giving a proton instead.

Feynman diagram for beta decay: (at the quark level)


So far the proton hasn’t been seen to decay.

An antiparticle can be thought of as a particle going back in time.

The weak force is here mediated by W exchange. The weak force only looks weak because the W is such a heavy particle ≈ 80 GeV (1983)

It only acts over a short distance which is why it will only work inside the proton and neutron.

The photon and the gluon are both massless. Why are the W and Z bosons not massless also?

Ans: the W and Z bosons get their masses in the theory via their interaction with the Higgs field!!!

The Higgs field is scalar.

Zo boson can decay to an electron and positron. A Z boson can also decay to a larger particle.

Fermions are matter particles.

Ambient Higgs field behaves like a ferromagnet.

The LHC was needed to discover the Higgs.

W→ e ѵ decay in seen in the ATLAS experiment



1973 Discovery of “Neutral Currents” in the Gargamelle Bubble Chamber at CERN (implies existence of Z boson)


A neutral current event observed in the Gargamelle bubble chamber at CERN. The charged particles are deflected by magnetic fields.

1983 Discovery of the W and Z bosons in the UA1 Experiment at CERN W± ≈ 80 GeV Z0 ≈ 91 GeV (this was the largest particle known at the time) (W and Z masses due to interaction with the Higgs field!)

1994 Discovery of the “top” quark in the CDF and D0 Experiments at FNAL mtop ≈ 175 GeV (This is the heaviest particle to date) (top quark mass from interaction with the Higgs field!)

2012 Discovery of a boson believed to be the Higgs

2013 Boson is the Higgs. Peter Higgs and Francois Englert awarded the Nobel Prize for physics.

Feynman diagram involving the Weak “Neutral Current” force



The Higgs is still being investigated.

The 4 LEP Experiments at the LEP e+ e collider at CERN (Geneva)

LEP was built to study the Z




The peak is at 91.1876 GeV

There are 12 “matter” particles (fermions)


Identical fermions obey the “Pauli Exclusion Principle”

The Pauli Exclusion Principle is the quantum mechanical principle that two identical fermions (particles with half-integer spin) cannot occupy the same quantum state simultaneously. In the case of electrons, it can be stated as follows, it is impossible for two electrons of a poly-electron atom to have the same values of the four quantum numbers (n, ℓ, mℓ and ms). For two electrons residing in the same orbital, n, ℓ, and mℓ are the same, so ms must be different and the electrons have opposite spins. This principle was formulated by Austrian physicist Wolfgang Pauli in 1925.

The muon appears to live longer. Neutrinos can interact with ice. The top quark is not produced in Z decay.

The discovery of the top quark at Fermilab 1994 (USA)

The very heavy top quark mass is “explained” in the theory by saying that the top quark has a very large coupling (interaction) with the Higgs field !!! (significantly larger than that of the W and Z bosons)

All the other “matter” particles have much smaller couplings to the Higgs field and hence much smaller masses e.g.

“bottom” quark (b) ≈ 5 GeV

“charm” quark (c) ≈ 2 GeV

“strange” quark (s) ≈ 0.1 GeV

electron = 0.5 MeV etc.

neutrino = 50 meV etc.


The mass of the top quark is » 175 GeV

Interaction with the ambient all-pervasive Higgs field gives mass to the fundamental particles:




The Higgs field is non-zero even in a vacuum. Interaction with the non-zero Higgs field gives masses to the fundamental particles. Waves in the Higgs field correspond to a new kind of “force” particle: The Higgs boson!!

The LHC discovered this particle


Feynman diagram for W pair production at LEP2 (“electroweak” theory)




In particle physics and physical cosmology, the Planck scale (named after Max Planck) is an energy scale around 1.22 × 10^19 GeV (which corresponds by the mass–energy equivalence to the Planck mass 2.17645 × 10^−8 kg) at which quantum effects of gravity become strong. At this scale, present descriptions and theories of sub-atomic particle interactions in terms of quantum field theory break down and become inadequate, due to the impact of the apparent non-renormalizability of gravity within current theories.

At the Planck scale, the strength of gravity is expected to become comparable with the other forces, and it is theorized that all the fundamental forces are unified at that scale, but the exact mechanism of this unification remains unknown. The Planck scale is therefore the point where the effects of quantum gravity can no longer be ignored in other fundamental interactions, and where current calculations and approaches begin to break down, and a means to take account of its impact is required.

We may be getting close to realising Einstein’s dream that there is a method of unifying all the fundamental forces (electromagnetic, strong and weak forces) with gravity.

If you want to find the boson look for the fermions and if you want the fermions then look for the boson.

The grand unification energy LGUT, or the GUT scale, is the energy level above which, it is believed, the electromagnetic force, weak force, and strong force become equal in strength and unify to one force governed by a simple Lie group. Specific Grand unified theories (GUTs) can predict the grand unification energy but, usually, with large uncertainties due to model dependent details such as the choice of the gauge group, the Higgs sector, the matter content or further free parameters. Furthermore, at the moment it seems fair to state that there is no agreed minimal GUT.

The exact value of the grand unification energy (if grand unification is indeed realised in nature) depends on the precise physics present at shorter distance scales not yet explored by experiments. If one assumes the Desert and supersymmetry, it is at around 10^16 GeV.

In particle physics, supersymmetry (SUSY) is a proposed extension of spacetime symmetry that relates two basic classes of elementary particles: bosons, which have an integer-valued spin, and fermions, which have a half-integer spin. Supersymmetry may be the way forward in incorporating gravity into the standard model and unifying the forces.

We shouldn’t talk about force and matter particles but instead about fermions and bosons.

Matter particles are fermions because they obey the Pauli Exclusion Principle. They exclude each other

Force particles are bosons. They attract each other. Stimulated emission by lasers etc.

Will supersymmetry (SUSY) indicate a symmetry between fermions and bosons?

Hands on computer workshop to illustrate the principle of the Large Hadron Collider experiments

You can download the activities from

The activity was about how to detect particles.

The ATLAS detector consists of four major components; inner detector (yellow) – measures the momentum of each charged particle; calorimeter (orange and green) – measures the energies carried by the particles; muon spectrometer (blue) – Identifies and measures the momenta of muons; magnet system (grey) – bending charged particles for momentum measurement.


The track detector’s basic function is to track charged particles by detecting their interaction with material at discrete points, revealing detailed information about the types of particles and their momentum.

The electromagnetic (EM) calorimeter absorbs energy from particles that interact electromagnetically, which include charged particles and photons. It has high precision, both in the amount of energy absorbed and in the precise location of the energy deposited.

The hadron calorimeter absorbs energy from particles that pass through the EM calorimeter, but do interact via the strong force; these particles are primarily hadrons.

The students looked at different ATLAS cross-sections and using the tracks to identify the particles and the events that caused them.



First candidate for an event with a Z boson decaying to two muons seen in 7 TeV collision data 2010



ATLAS event containing four muons

This event is consistent with coming from two Z particles decaying: both Z particles decay to two muons each. Such events are produced by Standard Model processes without Higgs particles. They are also a possible signature for Higgs particle production, but many events must be analysed together in order to tell if there is a Higgs signal. This view is a zoom into the central part of the detector. The four muons are picked out as red tracks. Other tracks and deposits of energy in the calorimeters are shown in yellow.

In the picture below Matthew, Aslam, Wing Chung and Alfie are listening to the instructions for the task.


Matthew, Aslam and Pameer (towards the back) starting the activity


Aslam, Wing Chung and Pameer

Dr. Emmanuel Olaiya overseeing the activity



Wing Chung and Pameer getting some advice


Aslam and Matthew analysing events to enable them to identify the particles involved

The picture below shows the spreadsheet for pooling results.



Aslam and Matthew collecting their prize for discovering the Higgs Boson Unfortunately it wasn’t a Nobel Prize (apologies for the dreadful photography)

After a delicious lunch our afternoon began with a lecture on the large hadron collider by Dr Kristian Harder.

The Large Hadron Collider

Dr Kristian Harder


What is the status of particle physics at the moment?

We have an excellent description of particle physics at the moment. It is provided by the standard model with unprecedented precision.


However there are two problems, it is incomplete and partly wrong.

a) Incomplete: How do we explain gravity? It is too weak to affect individual particles. How do we explain dark matter and dark energy? How do we explain mass?

b) Wrong: There are mathematical inconsistencies at higher energies (i.e. the Standard Model is a low energy approximation); Some things just don’t feel right • so many free parameters • so many different mass scales

Hopefully the question of mass will be solved with the Higgs Particle.

In the current theory, forces are mediated by particles (photon, gluon, W, Z) and the mathematics only works if the force particles are massless. Unfortunately some are not.

Potentially explained by the Higgs mechanism proposed in the 1960s by theoretician Peter Higgs and others: maybe massive particles only appear massive due to some background interaction?

An analogy to explain the Higgs mechanism

Put simply, the Higgs boson is thought to be the elementary particle responsible for the existence of mass.

The concept was introduced into particle physics in the 1960s as a means of solving the problem of why some force-carrying particles have mass but others don’t.

In the “Standard Model” of particle physics, the electromagnetic force is carried by photons, which are familiar to us as particles of light, and the weak nuclear force is carried by particles called W+, W- and Z bosons.

Abdus Salam, Sheldon Glashow and Steven Weinberg found that the electromagnetic and weak forces are different manifestations of a single phenomenon – at an energy of around 100 GeV, they unify into what is known as the electroweak force. However it was not understood why the photon is massless and the W and Z particles are massive.

What is now known as the Higgs mechanism was proposed by Peter Higgs and others as a way of explaining why this should be the case.

The Higgs field is a background field permeating space, the Higgs boson is a field quantum!

In the Standard Model, quantum numbers such as electric charge are dependent on ‘coupling’ to the appropriate field – in this case the electromagnetic field. The Higgs mechanism introduces another – the Higgs field – into the theory. Particles that couple to this field gain a mass, while those that don’t couple to it will remain massless.

The Higgs particle is a quantum of the Higgs field in a similar sense to a photon of light being a quantum of an electromagnetic field.

There are also some versions of the Higgs mechanism in which there is a field but no particle.

The Higgs Boson is a force and the Higgs field is everywhere.

It was suggested that if the Higgs mechanism is real we should see an extra massive particle. The LHC was built to produce enough energy to do this.

On the 4th July 2012 ATLAS and CMS physicists observed a new massive particle that might be a Higgs. But it might not have been the Higgs. It could have been one of many Higgs, but which one?

The LHC was not built to —————


Create black holes to swallow Earth


Open a stargate for the return of Satan


Create antimatter bombs to destroy the Vatican (Angels and Demons)

The best way to understand matter is to break it up. To do this we need to smash stuff together at high speed and that is what the LHC does.

The speed is the important part and for this you need accelerators.


If you collide two cars together you will simply get bits of car. However if you collide two protons together at high speeds (with enough energy – E = mc^2) you can create new particles never seen before.



(Left) in a proton-proton or proton-anti-proton collision, a bottom quark (“b quark” for short) is created; (Centre) a bottom hadron forms around the bottom quark; (Right) After traveling a great distance (relative to the previous panels), a fraction of a millimetre, the bottom quark decays (inset) to two lighter quarks and an antiquark, and this causes the bottom hadron to fragment into many lighter hadrons, none of which are bottom hadrons.

One of the first accelerators was created by J.J. Thomson


Sir Joseph John Thomson, OM, FRS (18 December 1856 – 30 August 1940) was a British physicist.

In 1897 he showed that cathode rays were composed of a previously unknown negatively charged particle, and thus he is credited with the discovery and identification of the electron; and, in a broader sense, with the discovery of the first subatomic particle. His apparatus involved accelerating the particles.



The diagram above appeared in an article by J.J. Thomson in 1897 announcing the discovery of the electron. The long glass finger (in the photo) projecting downward from the right-hand globe is where the entire tube was evacuated down to as good as a vacuum as could be produced, then sealed.

The two plates about midway in the CRT were connected to a powerful electric battery thereby creating a strong electrical field through which the cathode rays passed (this was the part of the equipment causing the acceleration). Thomson also could use magnets, which were placed on either side of the straight portion of the tube just to the right of the electrical plates. This allowed him to use either electrical or magnetic or a combination of both to cause the cathode ray to bend.

Thomson was awarded the 1906 Nobel Prize in Physics for the discovery of the electron and for his work on the conduction of electricity in gases.

Over the years accelerators have got more powerful (and bigger)

University-/Lab-scale experiment

The Bevatron was a 6.5 GeV particle accelerator — specifically, a weak-focusing proton synchrotron — at Lawrence Berkeley National Laboratory, U.S.A., which began operating in 1954. The antiproton was discovered there in 1955 (and the antineutron in 1956), resulting in the 1959 Nobel Prize in physics for Emilio Segrè and Owen Chamberlain. It accelerated protons into a fixed target, and was named for its ability to impart energies of billions of eV (Billions of eV Synchrotron).


Edwin McMillan and Edward Lofgren on the shielding of the Bevatron. The shielding was only added later, after initial operations.

The LHC is much bigger – it is the largest experiment ever built!


The objective of the experiment is to smash particles together, protons for example


1. Get a bottle of protons (use hydrogen and ionise it!) use them sparingly: one LHC fill has 2 beams × ≈3000 bunches × 10^11 protons, i.e. about 1 nanogram, which should circulate ≈ one day

2. Keep your protons in vacuum pipes at all times so your protons don’t get disturbed too much. LHC: 1/10,000,000,000,000th of atmospheric pressure (better vacuum than space around the Intl. Space Station)!

3. Accelerate your proton beams with electric fields. LHC: protons will achieve ≈ speed of light, total kinetic energy of proton beam: Eurostar train at ≈100 mph!

You cannot get that much energy from one pass through the accelerating cavities! You will have to bend the beam around with magnetic fields and accelerate it repeatedly. Do you think you could force a 100 mph Eurostar onto a circle using only magnets?

You would need very strong magnets and a large circle (9 km diameter!)

LHC Particle Acceleration In-depth Explanation


4. Use strong magnets to steer the proton beams strong magnets require huge currents — only manageable with superconducting magnets! LHC is like the largest fridge on the planet! 6000 tons kept at -271◦C corresponding to ≈150,000 household fridges at a temperature colder than the coldest regions of outer space!


There are two beams of protons circulating in opposite directions, each with ≈3000 bunches of 10^11 protons usually in separate pipes, but crossing each other in 4 places ≈ 20 collisions every 25 ns! You need gazillions of collisions because the interesting things might only happen once per billion or trillion collisions!


Collisions occur in 4 places and are investigated by big detectors


The four large LHC detectors are:

ATLAS (general purpose) 7000 tons, 25m diameter, 46m length 2500 scientists & engineers

CMS (general purpose) 14500 tons, 15m diameter, 22m length 3000 scientists & engineers

LHCb (b physics) 5600 tons, 13m width, 21m length 700 scientists & engineers

ALICE (heavy ion physics) 10000 tons, 16m diameter, 26m length 1000 scientists & engineers

Protons smash into each other at specific locations in the LHC. This creates interesting massive particles, which decay almost immediately (only a few mm further out from where the collisions happen). Unfortunately the decay products do fly in all directions (not all of them interesting).


The decay products are intercepted and analysed with detectors and the information is used to reconstruct the interesting parts of the event mathematically using computers & brains.

Another set of problems are that neutrinos are undetectable quarks and gluons never appear isolated. The collisions always give rise to whole bundle of protons, pi mesons, neutrons etc. (a jet).

Charged particles ionise materials they pass through. If a small amount of material is put in their way and an ionisation charge is detected in there and you can localise where the ionisation happened then you can follow the path of charged particles almost without disturbing them! Typically layers of silicon detectors are used because: they are quite thin (few hundred micrometer) they high resolution (few micrometer) they are radiation hard (survive LHC collisions for years) technology enables them to be mass produced → cost benefits



The ionisation can also cause a current to flow which can be measured.

Another trick to derive information about charged particles is to immerse the tracking detector in a magnetic field causing the path of particle to get bent. You can then calculate particle momentum from the amount of curvature!

BUT: the high energy collider produces high energy particles which results in a small curvature! You need a large tracking detector (few meter flight distance) and high spatial resolution (few micrometres) to get a useful measurement!


The tracking detectors cover electrons, muons, protons and charged pions because they are charged particles, but what about the neutral particles such as photons, neutrons and neutral pions.

You need to use a complementary detector type such as a calorimeter!


After the particles have passed the tracking detectors a massive amount of material is placed in their path. The particles lose energy due to material interaction and showers of secondary particles are produced.

Absorbers are used to stop the particle and the kinetic energy is transformed into the energy of the shower (ionisation, light, number of charged particles,…..). You can derive the energy of the particle from the energy of the shower (and get flight direction from location of the shower).

There are two main classes of calorimeters: In an electromagnetic calorimeter absorption occurs via an electromagnetic cascade of lightweight particles (electron, photon):



Hadronic calorimeter involves a nuclear interaction with an absorber but high energy particles need a very thick material (few meters) of high density such as iron, lead and even uranium.

Observing particle energy and momentum is not enough we also need to know the particle type.

The solution is to use a tracker, electromagnetic calorimeter and a hadronic calorimeter together but this doesn’t stop the muons. Muons are almost unstoppable.

Muons are a special case because they are too heavy to develop electromagnetic showers or nuclear interactions. There is no hadronic shower. They could be identified from feeble signals in calorimeters but this is not very reliable. You need to use yet another detector type behind the tracker & calorimeters:


Often a coarse tracker is used to identify muon direction without help of the main tracking detector. It can even have its own magnets to measure the muon momentum independent of the tracker! Now muons are easy to identify because whatever gets through the calorimeters and is visible is probably a muon.

The tools mentioned so far can identify almost all particles we can catch but there is one important exception. Neutrinos cannot be detected. Also there might be unknown particles that do not interact with normal matter is there any way their presence can be tagged?

Neutrinos can’t be seen but the momentum they carry away can be measured.

The law of conservation of momentum conservation means that before the collision there is zero momentum perpendicular to the beams and after the collision there must also be zero net momentum perpendicular to the beams!

When the momenta of all visible particles are added up is there an imbalance? If so then this might be the momentum carried away by a neutrino!

The missing momentum identification requires our detectors to be “wrapped around” the interaction region! (“hermetic detector”)


The ATLAS detector has a diameter of 25m, a length of 46m, about 70 m^2 of silicon detectors and a mass of 7000 tonnes.

The detector and all its subsystems need to be monitored by experts around the clock. Its control room is close to the detector, but it could be anywhere.

Reading data from ATLAS involves 100 million individual readout channels (pixels, cells, modules, …) and signals need to be digitised for computer processing using lots of specialised electronics and 3000 km of cables.



This event was detected at the LHC’s CMS experiment, in which one Z boson particle decays to two electrons (red towers), and another Z boson decays to two muons (red lines). Such an event is a candidate event for the Higgs boson particle.

More evidence that the Higgs Boson was discovered was a W Boson decaying to μvm


So who volunteers to browse through 40 million events per second?


• fast special electronics that identifies interesting signals

• selection in several stages of increasing complexity and precision

• higher levels done by PC farms

• ≈ 100 events per second stored permanently (filling ≈ 1 CD per second), distributed all over planet for computerized reconstruction and analysis.

Only the “boring” particles show up in the detector, e.g. the muons and electrons, but not the Higgs that decayed into them! So how is reconstruction of what happened before done?

A simple example involves a Z Boson decaying to a positive muon and a negative muon. You need to find events with two muons and assume that these muons came from a Z decay. Calculate the mass of the supposed Z from muon momenta and if you get consistent results for the mass you can say you have found the Z If not then these muons do not come from Z Bosons.

Black holes at the LHC

If gravity becomes unusually strong at small distances (e.g. related to extra dimensions) then the LHC energy might be enough to create tiny black holes in collisions! Small black holes decay very quickly (Hawking radiation). What does black hole decay look like? Probably many high energy particles with a thermal distribution of flavours,

We know these black holes (if they exist) won’t eat Earth! High energy cosmic rays haven’t so we won’t!

A planetary scale experiment


CERN accelerators


Particle Physics Benefits: Particle physics advances technology on all fronts; Medical applications (MRI, for example); The world wide web; GRID computing and other forms of massive data processing.

Understanding how the Universe works: Quantum physics has led to the laser, advances in medicine, machinery and even DVD players; Theory of relativity has led to GPS; Synchrotron radiation has many uses such as structural analysis of crystalline and amorphous materials, energy dispersive X-ray diffraction, powder diffraction analysis, X-ray crystallography of proteins and other macromolecules, magnetic scattering, small angle X-ray scattering, X-ray absorption spectroscopy, inelastic X-ray scattering, soft X-ray emission spectroscopy, Tomography, X-ray imaging in phase contrast mode, X-ray standing wave experiments, photolithography for MEMS structures as part of the LIGA process, high pressure studies, residual stress analysis, X-ray multiple diffraction, photoemission spectroscopy and angle resolved photoemission spectroscopy.

A synchrotron light source is a source of electromagnetic radiation (EM) usually produced by a storage ring, for scientific and technical purposes. First observed in synchrotrons, synchrotron light is now produced by storage rings and other specialized particle accelerators, typically accelerating electrons. Once the high-energy electron beam has been generated, it is directed into auxiliary components such as bending magnets and insertion devices (undulators or wigglers) in storage rings and free electron lasers. These supply the strong magnetic fields perpendicular to the beam which are needed to convert the high-energy electron energy into photons.

The major applications of synchrotron light are in condensed matter physics, materials science, biology and medicine. A large fraction of experiments using synchrotron light involve probing the structure of matter from the sub-nanometre level of electronic structure to the micrometre and millimetre level important in medical imaging. An example of a practical industrial application is the manufacturing of microstructures by the LIGA process.


Synchrotron radiation reflecting from a terbium crystal at the Daresbury Synchrotron Radiation Source, 1990

Tour of Diamond light source and ISIS

Unfortunately we had to leave before we had a chance to go on the tour.


One thought on “Particle Physics Masterclass 2014

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